Japanese Patent Laid-Open No. 2002-89320 discloses technology (hereunder, referred to as “prior art”) relating to air-fuel ratio switching control in an internal combustion engine that is capable of switching the operational air-fuel ratio of the internal combustion engine between a lean air-fuel ratio and a stoichiometric air-fuel ratio. According to the aforementioned prior art, when switching the air-fuel ratio from the lean air-fuel ratio to the stoichiometric air-fuel ratio, operations are performed so that the air-fuel ratio is gradually switched after starting operation of a throttle valve. Since switching of the air-fuel ratio is thereby performed in correspondence with control of the throttle valve, variations in the fuel control are suppressed and fluctuations in the engine output torque are prevented.